1. Field of the Invention
The present invention relates to a motor-driven supercharger having a built-in electric motor.
2. Description of the Related Art
To supply an air having a density increased by a compressor to an engine is called supercharging, and an apparatus for performing a driving work of the compressor on the basis of an exhaust energy of an engine in the case of executing the supercharging is called as an exhaust-gas turbocharger.
The exhaust-gas turbocharger is generally constituted by a compressor and a turbine which are arranged so as to sandwich a bearing unit therebetween. The compressor has a built-in compressor impeller, and the turbine has a built-in turbine impeller. The compressor impeller and the turbine impeller are coupled to each other by a connecting shaft (a shaft) supported by a bearing unit, and are structured such as to rotationally drive the turbine impeller on the basis of an exhaust gas of the engine, transmit the rotating force to the compressor impeller via the connecting shaft, and compress the air by the compressor impeller so as to supply the compressed air to the engine.
In the exhaust turbine supercharger mentioned above, there has been already proposed a structure having a built-in electric motor for assisting an acceleration at a time of rotating at a low speed (for example, patent documents 1 and 2). Hereinafter, the exhaust-gas turbocharger having the built-in electric motor is simply called as “motor-driven supercharger”.
Further, patent document 3 is proposed as an air cooling system of the motor-driven supercharger.
As shown in FIG. 1, a turbocharger in the patent document 1 is provided with an electric motor (a stator 51 and a motor rotor 52) rotationally driving a rotor. The rotor is constituted by a turbine impeller 53, a compressor impeller 54, a shaft 55 coupling them and a spacer collar 56 to which the motor rotor 52 of the electric motor is attached. The shaft 55 is supported by a single semi-floating bearing 57. The semi-floating bearing 57 has a thrust surface 57a brought into contact with a hub 53a of the turbine 53, and a thrust surface 57b brought into contact with the spacer collar 56.
In accordance with this structure, a thrust force applied to the rotor is supported by a housing 50 via the thrust surfaces 57a and 57b of the semi-floating bearing 57 respectively engaging with the hub 53a of the turbine and the spacer collar 56.
As shown in FIG. 2, the motor assist supercharger in the patent document 2 has a motor rotor 62 supported by an extension portion 61a in an axial direction of a compressor wheel 61, and a stator 64 supported by a housing 63 so as to be spaced to an outer side in a radial direction from the motor rotor 62.
In the air cooling system of the patent document 3, as shown in FIG. 3, a turbocharger 72 coupled to an internal combustion engine 71 has an electric motor accommodated within a motor housing 73, the motor housing 73 has outlet and inlet ports 73a and 73b. The turbocharger 72 further has a compressor 74 and outlet and inlet ports 74a and 74b. Further, a first cooling hose 75 couples the compressor outlet 74a and the motor housing inlet 73b via an intercooler 76, and a second cooling hose 77 couples the motor housing outlet 73a and the compressor inlet 74b so as to recirculate an air heated by cooling the electric motor to the compressor 74.
Patent Document 1: U.S. Pat. No. 6,449,950 “ROTOR AND BEARING SYSTEM FOR ELECTRICALLY ASSISTED TURBOCHARGER”
Patent Document 2: U.S. Pat. No. 6,032,466 “MOTOR-ASSISTED TURBOCHARGERS FOR INTERNAL COMBUSTION ENGINES”
Patent Document 3: U.S. Pat. No. 6,609,375 “AIR COOLING SYSTEM FOR ELECTRIC ASSISTED TURBOCHARGER”
In the motor-driven supercharger, since a stator (a motor stator) and a rotor (a motor rotor) of a built-in electric motor generate heat by an eddy current or the like, it is necessary to efficiently cool them.
However, according to the patent document 1, in the case that the stator 51 is gripped by two electric motor casings 58a and 58b from both sides in an axial direction, the stator 51 is cooled only by a natural heat dissipation via the casings 58a and 58b. Accordingly, there is a problem that a cooling performance is low.
Further, in the example of the patent document 2, the casing 63 is provided with a water cooling jacket 66 coming into contact with an outer peripheral surface of the stator 64. However, since a heat input from the compressor wheel 61 and the motor rotor 62 which are heated is large, it is impossible to efficiently cool a whole of the stator.
Further, in the example of the patent document 3, since a compressed air cooled by the intercooler 76 is supplied into the motor housing 73, it is possible to efficiently cool an inner side of the motor housing. However, since a part of the compressed air is used in a circulating manner for cooling, an amount of the compressed air supplied to the engine is reduced, and an apparent efficiency of the compressor is lowered.